Currently, some polymers, such as synthetic plastics derived from fossil fuels (e.g. petroleum), are unable to be degraded. So, the polymers are accumulated semipermanently in natural environment, resulting in various environmental problems. Amid growing public concerns over recent environmental problems, new polymers are being developed from recyclable sources that are not derived from fossil fuels.
Under these circumstances, much academic attention has been focused on polyhydroxyalkanoate (PHA), which is produced from reproducible biological organic resources (biomass) such as sugar and vegetable oil by fermentation method, as a biodegradable plastic (Green Plastics, known as eco-friendly polymer) having thermoplasticity and excellent biodegradable and biocompatible properties. Such biodegradable plastics are currently developed in technical properties toward practical use, and are expected to become leading biomaterials in biological and medical fields.
PHA is conventionally known as a polyester-type organic molecule polymer that can be accumulated in a microbial cell body in many types of microorganisms, as disclosed in Japanese Unexamined Patent Application Publication No. 2006-320256 (a microorganism capable of producing polyhydroxybutyrate (PHB) that belongs to genus Methylocystis that can accumulate PHB in a microbial cell body).
Meanwhile, a PHA that is produced by a microorganism and accumulated in a microbial cell body thereof is found to include 90 types or more of monomer structures (see FEMS Microbiol. Lett., 1995, 128219 to 128228). In PHA, properties are significantly affected by its constituting monomer units, like 3-hydroxybutyrate (3HB), from which a typical PHA is produced by a microorganism. However, such a PHA is of high crystallinity and low flexibility, resulting in limited industrial uses.
In order to obtain a PHA having monomer units other than 3-hydroxybutyrate (3HB), the following operations are required: changes in the type of PHA-producing microorganism, culture medium composition and culture conditions, and the isolation of microorganism and use of transformants transformed by gene recombinant techniques. For example, Japanese Unexamined Patent Application Publication No. 2001-178484 discloses a method for producing a PHA including at least one of 6 numbers of carbon atoms (hereinafter called C6), C8, C10, C12 and C14 monomer units in Pseudomonas cichorii YN2 cultured in a culture medium, by assimilating acetic acid or its salt as a sole carbon source. Another Japanese Unexamined Patent Application Publication No. 2004-321167 discloses a method for producing a PHA comprising C6 to C10 monomer units by culturing a specific transformant in a culture medium including C6 to C10 as carbon sources.
A ratio of monomer units in a PHA varies according to substrate specificity of PHA synthase and monomer synthetic pathway in a host. PHA synthases are classified into four groups (types I to IV) according to primary structure and substrate specificity. Type I and III PHA synthases show substrate specificity in a hydroxyacyl-CoA of short-chain length (about C3 to 6), type II PHA synthase shows substrate specificity in a hydroxyacyl-CoA of medium-chain length (about C6 to 12), and Type IV PHA synthase shows substrate specificity in a hydroxyl-CoA of short to medium-chain length (about C4 to 8). Due to a dependence of properties on monomer composition in PHA, the production of versatile PHA requires PHA synthase having various substrate specificities.
Meanwhile, Japanese Unexamined Patent Application Publication No. 2002-335966 discloses a strain that produces poly-3-hydroxybutyrate at about medium temperature (40 to 50° C.) and belongs to bacillus (bacilliform bacterium) sp., a PHA synthase, a gene encoding the same, etc. By culturing the strain or using the PHA synthase obtained, a PHA with high thermal stability can be stably produced even at about medium temperature.